Influence of Nut Development on Pecan Nut Casebearer Damage in Pecans

Guide H-650

College of Agriculture, Consumer and Environmental Sciences New Mexico State University.(Print Friendly PDF)

This publication is scheduled to be updated and reissued 7/05.

Environmental conditions, management inputs, and pest pressure all
have direct impacts on pecan yield or nut quality. In many instances, the
physiological stage of the pecan nut determines if a specific adversity
(environmental, management, pests) will result in reductions in yield,
quality, or both.

This guide covers the development of a pecan nut throughout the season
and how it relates to potential damage by pecan nut casebearer. Because
of uncertainty related to third-generation PNC damage in New Mexico pecan
orchards, a brief discussion on possible damage is included.

Pecan Nut Development in the Mesilla Valley

Nut production begins with the onset of flowering. Pollen shed from catkins
(male flowers) generally is abundant every year. Female flower structures
may be present in adequate numbers or they may be scarce if the crop was
exceptional the previous year and alternate bearing is taking place in
the orchard.

Pollination is the first stage in the development of a pecan nut. The
remaining stage can be subdivided into two major phases, I and II. Because
of variations in environmental conditions, it is difficult to place an
exact date on the initiation or completion of phases or stages of nut development.

Phase I begins at pollination (early to
mid-May) and lasts until the initiation of shell hardening (mid-August).
During this 90-day period, nuts enlarge to their final size. A period of
fast nut growth starts about mid-July and ends in late August. Liquid content
inside pecans makes irrigation management critical in pecan orchards during
this time. Maximum water stage (content) occurs from mid-August to the
end of August (fig. 1). During this stage, nut drop could dramatically
intensify due to water stress.

Figure 1. Relative nut growth rates and average nut development
stages for 'Ideal' ('Bradley') and Western Schley' pecans in the Mesilla
Valley, New Mexico.

Phase II begins at shell hardening and concludes
at nut maturation in late October. During this six-week period, kernel
development and nut filling occurs. During phase II, the liquid within
the kernel begins to thicken (thus the name "gel stage") around the third
week in August.

Approximately 7 to 12 days after the initiation of the gel stage, the
dough stage begins. The dough stage is characterized by the absence of
liquid or gelatinous material within the nut. Nut maturity soon follows
with the splitting of the four sutures on the hull and exposure of the
shell. Moisture in the nut may drop from 30 to 12 percent during this period.
Release of the nut from the dried shuck depends on climatic factors and
on the nut shape. High environmental moisture and round nut shapes both
contribute to increase in nut retention by the husk.

Pecan Nut Casebearer (PNC) in the Mesilla Valley

Pecan nut casebearer is not new to New Mexico pecan production. It has
been a problem in the eastern pecan-growing region of the state for the
past 30 years. Natural borders created by Franklin, Oregon, Sacramento,
and Guadalupe Mountains and the vast expanse of nonirrigated desert have
prevented its natural migration from the east to the western pecan-growing
regions.

In 1988, pecan nut casebearer was found on homegrown trees in El Paso,
Texas-the first time this pest was recorded in the western growing region.
By 1994, PNC had moved north from El Paso to pecan-growing areas in New
Mexico's Mesilla Valley. Currently, PNC has established itself in over
70 percent of the pecan-producing areas in the western region of New Mexico.
Within the next four years, it is estimated that PNC will establish in
the majority of the remaining pecan acres.

Seasonal PNC moth activity in the western pecan growing regions of New
Mexico, as monitored by pheromone traps, is found in fig. 2. Three distinct
moth flights are evident. During 1988, the first flight began the middle
of May, with moths emerging from the overwintering larval population. The
second moth flight began in early July, with the third flight beginning
toward the end of August.

Figure 2. Mean number of pecan nut casebearer moths for combined
locations during 1998 in the Mesilla Valley, New Mexico.

Using pheromone trap results combined with basic information on PNC
biology and field observations, a reasonable estimate of when PNC larvae
are present can be obtained (fig. 3). Number of larvae (fig. 3) is relative
to number of moths caught in pheromone traps and is not intended as an
estimate of field populations. The value of fig. 3 is found in the estimated
periods when PNC larvae were present during 1998.

First-generation larvae began feeding on nutlets toward the end of May
and continued feeding until the middle of July. Second-generation larvae
began feeding in mid-July and continued until early September. By the time all second-generation larvae had
completed development, third-generation larvae were present. During 1998,
this generation overlap was particularly evident. Both late instar second-generation
and early instar third-generation larvae were collected concurrently in
'Western Schley' pecan orchards.

First- and early second-generation PNC feed during phase I of nut development.
This phase represents the period from pollination or soon after to shell
hardening. Crop damage by PNC larvae is restricted to yield reduction during
this period. Larvae are able to feed through the husk tissue and into the
interior of the nut. Nut damage results in the formation of an abscission
layer within the peduncle, and the eventual dropping of the nut from the
cluster. Depending on the nut load, first- and early second-generation
PNC feeding may thin a heavy crop load or significantly reduce orchard
yield potential.

Nut damage due to second- and early third-generation PNC larvae depends
on the developmental stage of the nut (table 1).

Larval feeding prior to hard shell (mid-August) typically results in
significant damage to the nut, formation of an abscission layer, and loss
of the nut. It is important to remember that nuts develop at different
rates. During the period of the pecan's shell hardening (three weeks) a
percentage of nuts are susceptible to yield loss caused by late second-
and early third-generation PNC larvae. Crop loss during the late second
generation can be particularly damaging to final yield. This period of
time is after August drop, so affected nuts represent direct yield loss.
Damaged nuts before August drop represent potential yield loss.

After completion of hard shell (early August), third-generation PNC
dominate in southern New Mexico pecan orchards. Although PNC cannot penetrate
the shell, larvae feed and tunnel freely within the husk, along the shell
(grazing), and at the point of attachment between the husk and the peduncle.
The nut filling process takes approximately six weeks in the Mesilla Valley
and is usually complete by mid-October. During this period, nutrients and
water move through the peduncle and husk and into the developing nut. Damage
due to PNC feeding during this period does not normally result in a loss
of a nut but can possibly reduce nut quality.

During the 1998 growing season, evidence of PNC larvae grazing along
the exterior of the shell and feeding within the husks was prevalent in
orchards with high third-generation larval populations. Of those nuts extensively
fed on by third-generation larvae, a limited number exhibited quality problems
directly associated with larval feeding. These problems manifested themselves
either in the form of shriveled meats in a localized area or darkened meats
directly below areas where shell grazing occurred.

Preliminary research conducted during 1998 indicates nuts are susceptible
to significant reductions in meat quality only during a short period after
shell hardening. If you consider the physiological processes occurring
during phase II, limited reductions in quality due to extensive husk feedings
is possible up to maturation. Although the impact on yield by third-generation
PNC may be insignificant, the presence of dark or shriveled meat in a grade
sample may bring a shipment down a grade.

It is apparent that at least in some years, all three generations of
pecan nut casebearer can cause economic damage in New Mexico and other
western region pecan orchards. Damage
includes potential yield loss by first-, second-, and early third-generation
larvae and potential reductions in quality by late second- and third-generation
larvae.

Mistimed or no insecticide applications directed at economic levels
of first- and second-generation larvae can contribute to late-season PNC
generation overlap. Overlapping generations can result in more difficult
timing of insecticide applications and an increase in the number of applications
than when distinct generations are present. Pecan nut casebearer developmental
stages can shift as many as 15 days from one year to the next. An effective
monitoring program for PNC and nut development during late phase I and
early phase II, combined with an effective suppression program, will help
ensure minimum pecan yield and quality reductions due to late-season generations.

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